Gas metal arc welding, commonly referred to as "GMAW" or "MIG" welding, is an arc welding process in which the arc is shielded from the ambient atmosphere by a gas. Metal is transferred to a workpiece through the arc from a consumable wire electrode. The consumable wire electrode is continuously fed into the arc at a preselected speed corresponding to a given deposition rate for a given wire size. The mode of metal transfer is dependent upon operating parameters such as welding current, voltage, wire size, wire speed, electrode extension and the protective gas shielding composition. The known modes of metal transfer include short circuit, globular transfer, axial spray transfer, pulse spray transfer and rotating arc axial spray transfer. Those skilled in the art are familiar with these terms and with their meanings. The short circuit, globular, and pulse spray modes of metal transfer are lower deposition rate processes in which metal is generally deposited at rates below about 10 pounds per hour (lbs/hr). The rotating arc axial spray mode of metal transfer is a very high deposition rate process which is unstable at metal deposition rates below about 17 lbs/hr using electrode wire size diameters of 0.035 inches or larger. Below this minimum rate of metal deposition the rotating arc becomes erratic and/or is unsustainable. The axial spray mode of metal transfer is used for depositing metal at rates above the low deposition rates provided by the short circuit, globular transfer or pulse spray modes of metal transfer and below the very high deposition rates obtained in the rotating arc axial spray mode.
A recent significant advance in the field of gas metal arc welding is the process described and claimed in U.S. Pat. No. 4,645,903--DeVito et al. which enables stable deposition in the axial spray mode of metal transfer up to a rate of 25 lbs/hr. using a wire diameter size within the range of from 0.035 to 0.052 inch. Prior to the advancement of this patent, the axial spray mode of metal transfer was limited to a metal deposition rate of from 5-12 lbs/hr. using the aforesaid wire sizes. Above the maximum deposition rate, the welding operation becomes erratic and less stable until a rotating arc is established by raising the wire feed rate to the minimum deposition rate required for a stable rotating arc.
The non-rotating arc axial spray mode of metal transfer is generally considered to be the preferred choice of metal transfer for all position GMAW welding where reliability, stability, high quality and at least a relatively high deposition rate is required. In the axial spray mode of metal transfer, fine droplets of molten metal are pinched off the end of the moving wire and fed axially through the arc column to the weld pool. The pinch effect is caused by electromagnetic forces on the molten tip of the wire. The rotating spray arc was developed to increase the operating deposition range for GMAW welding. In the rotating arc mode of metal transfer the arc is physically rotated by electromagnetic forces in a helical pattern about a longitudinal axis. As the arc rotates, a controlled stream of metal droplets is transferred from the electrode tip to the weld pool over a relatively wide area. The weld penetration provided by the non-rotating arc axial spray transfer mode is much deeper and is easier to control than that provided by the rotating arc mode.
It is thus desirable to have an improved spray mode gas metal arc welding process and it is particularly desirable to have an improved gas metal arc welding process which can extend the deposition range for metal deposition in the axial spray mode.
Accordingly it is an object of this invention to provide an improved spray mode gas metal arc welding process.
It is a further object of this invention to provide an improved gas metal arc welding process which can enable a significant increase in the rate at which metal may be stably deposited in the axial spray mode.